Story: Introduced animal pests

Page 3 – Rats and mice

New Zealand has no native rats, but three kinds came with
early sailors.

Kiore

Polynesians introduced the rat around 1250–1300 AD, when
they settled in New Zealand. Kiore are thought to have wiped
out snipe-rails, owlet-nightjars, some small petrels, some
native frogs, and all tuatara on the mainland.

Very few kiore now survive on the mainland as more
aggressive European rodents have replaced them.

Under the floorboards

European settlers were surprised by occasional swarms of
kiore, which were familiar to Māori:

‘In Picton, during the swarm of 1884, the stench
becoming unbearable in one of the houses, the floor of the
sitting room was removed, when forty-seven rats were found
lying together dead near the fireplace. ... Indeed, the
whole town was pervaded with the odour of dead rats. It
took the place of pastille in the drawing rooms, and
overpowered that of sanctity, even, in the
churches.’1

Norway and ship rats

Norway rats (Rattus norvegicus), also known as
brown or water rats, were on the ships of the first
explorers, who arrived in New Zealand in the late 1700s.
These rats quickly spread. Europeans brought ship rats
(Rattus rattus), also known as black or roof rats,
but these did not become established until after the
1860s.

Today, ship rats occur throughout the country, and are
abundant in kauri and rimu–rātā forest. They mostly live and
nest in trees. The spread of ship rats caused the sudden
decline of many native bird and bat species. For example,
when they were accidentally introduced to Big South Cape
Island (off Stewart Island) in 1964, they quickly eliminated
five types of native bird, one bat species, and a large
flightless weevil. In a study at Kōwhai Bush, near Kaikōura,
ship rats ruined and robbed eggs and nestlings from 16% of
small birds’ nests.

In the bush, ship rats feed mainly on fruit, berries and
fallen seeds in autumn and winter, and on other animals in
spring and summer. Among these are wētā, stick insects,
cicadas, beetles, caterpillars and grubs, spiders, native
slugs, snails and lizards.

Norway rats live from North Cape to Stewart Island. They
are ground dwellers, usually living near wetlands or in damp
lowland bush. They threaten animals living, roosting or
nesting near the ground, taking birds’ eggs and nestlings,
native insects and lizards.

House mice

Every bit of bush in the North and South islands harbours
house mice (Mus musculus), sometimes in plague
numbers. By eating insects and fallen seeds and berries, mice
deprive many native ground-feeding animals of food.

Beech trees produce heavy seed crops every two or three
years (‘masting’ years), providing a bonanza for mice whose
numbers often explode in response. Mice in turn are food for
stoats, whose numbers may also surge. Months later, when the
seeds have germinated, rotted or been eaten, and many of the
mice are also eaten, a hungry, increased stoat population
turns its predatory attention to native birds.